isolation rules of thumb
here to go to our main page on Wilkinson splitters
New for March 2007! Everyone
knows that you can get good isolation from a properly designed Wilkinson
from port 2 to port 3. Ideally the isolation is infinite at the
center frequency (if you have an odd number of sections)! But you
must also understand that this isolation is a function of the impedance
match that the common port sees. An ideal 50 ohm Wilkinson only
has perfect isolation when the ports are all perfectly terminated
in 50 ohms.
Let's look at an example, where
the common port sees a 1.5:1 VSWR. In the
Agilent ADS model below, this is provided by terminating port 1
with 75 ohms while the other ports are terminated in the proper
50 ohms. You can use our VSWR calculator
to see that 1.5:1 VSWR is equivalent to -13.98 dB return loss.
Now let's look at the response.
The isolation S23 is exactly -20 dB at the center frequency, while
the output match is also degraded to about -13.98 dB. It turns out
that -13.98 dB is equal to the equaivalent return loss of 1.5:1
VSWR that appears on the input port, so this mismatch appears in
all its glory at the split ports!
The center frequency isolation
is 6.02 dB worse than the return loss at the common port, and you
can verify yourself that this happens regardless of what VSWR you
place on the common port. So don't go design a Wilkinson for 20
dB isolation and place it after an amplifier that has a 2:1 output
match, you'll get around 16 dB isolation!
Time for two Microwaves101
rules of thumb!
The isolation of a Wilkinson is limited to 6 dB better that the
return loss of the source match at its common port.
The split port return loss of a Wilkinson is no better than the
return loss that is seen by the Wilkinson at its common port.